Optical filter

ABSTRACT

An optical filter that transmits light of the visible light region includes a dielectric substrate; a dielectric layer that is formed on a surface of the dielectric substrate; and a first metal structure group in which a plurality of first metal structures are arranged two-dimensionally in an isolated state in the in-plane direction of the dielectric substrate, that is provided between the dielectric substrate and the dielectric layer, comprising: the first metal structures having first and second lengths in first and second directions orthogonal to each other, which lengths are equal to or less than a first wavelength in the visible light region; and a transmittance of the first wavelength being reduced or a reflectance being increased by surface plasmons induced on a surface of the first metal structures by resonance between light incident on the dielectric substrate or the dielectric layer and the first metal structures.

This application is a division of application Ser. No. 12/665,562 filedDec. 18, 2009, which was the National Stage of International ApplicationNo. PCT/JP2008/063081 filed Jul. 14, 2008.

TECHNICAL FIELD

The present invention relates to an optical filter that utilizeslocalized surface plasmon.

BACKGROUND ART

Recently, hole-type optical filters in which apertures are periodicallyarranged in arrays on a metallic thin film and which perform wavelengthselection utilizing surface plasmons have been proposed in PatentDocument 1 and Non-Patent Document 1.

Conventionally, although also depending on the film thickness, thetransmittance of a metallic thin film including an aperture diameter ofa size that is less than or equal to a light wavelength is considered tobe less than approximately 1%.

However, as described in Patent Document 1, when apertures of apredetermined size are arrayed on a metallic thin film at a period thatis consistent with the wavelength of plasmons, the transmittance oflight of a wavelength that induces surface plasmons is significantlyenhanced.

Further, Non-Patent Document 1 teaches that transmission spectra of RGBcan be obtained using this kind of hole-type optical filter thatutilizes surface plasmons. More specifically, Non-Patent Document 1discloses that transmission spectra having wavelengths of 436 nm (blue),538 nm (green), and 627 nm (red) were obtained using a metallic thinfilm having subwavelength aperture arrays.

Patent Document 2 discloses a wavelength filter which utilizes a surfaceplasmon.

-   (Patent Document 1) U.S. Pat. No. 5,973,316-   (Patent Document 2) WO2002/008810-   (Non-Patent Document 1) Nature Vol. 424, 14 Aug., 2003 (FIG. 4)

DISCLOSURE OF THE INVENTION

In the above Patent Document 1 and Non-Patent Document 1, by using ametallic thin film having a relatively large surface area in which holesare periodically arranged, a filter is realized that has a transmissionspectrum depending on the wavelength of surface plasmons induced on themetal surface.

However, in this kind of hole-type metallic thin film filter, because alarge proportion of space is occupied metal, there is a large amount oflight absorption. Therefore, in the metallic thin film filter disclosedin the above described Patent Document 1, the transmittance is about 5to 6% even at a peak at which the transmittance is greatest.

When it is desired to utilize a transmission spectrum of this kind offilter that does not offer a very high transmittance, it is necessary toincrease the intensity of incident light in order to ensure theintensity of the transmission spectrum. Consequently, there is thepossibility that the energy efficiency of a device that uses a hole-typefilter will be low.

In particular, although the absorption of light by metal is not veryhigh in a microwave region, the light absorption of metal is high in avisible light region. Further, when a hole-type metallic thin filmfilter is used as a filter for a visible light region, the scope ofapplication to actual devices is narrowed.

Therefore, it is desirable to provide a high-transmittance opticalfilter in which light absorption is less than a hole-type metallic thinfilm filter in a waveband region containing a visible light region.

The filters described in the above Patent Documents 1 and 2 usesperipheral structures which have apertures or protrusions formed in ametal film having a relatively large surface area with pitchescorresponding to wavelengths of the surface plasmons to control theoptical properties. That is, the interference between the surfaceplasmons propagating along the peripheral structures selects the surfaceplasmon waves with their respective wavelengths corresponding to thepitches, and the selected waves then gather the intensities each otherto increase the intensity of a transmitted light and the intensity of areflected light.

Therefore, in the filters described in the above documents, the pitchesof the peripheral structures become a dominant factor defining theoptical characteristics of the filters. And when a wavelength isselected to obtain a certain optical characteristics, the pitches of theperipheral structures depend on the wavelength. That is, when a certainwavelength is selected, a density of the apertures or the protrusions inthe metal film might be restricted.

It is thus difficult to increase the transmittance or reflectance of thefilters.

And also, since the filters described in the above documents needperipheral alignment of the apertures or the protrusions, the size andthe surface area of the filter should be set to times the pitch. Thefilters of Patent Documents 1 and 2 thus have lower degree of freedomregarding the selection of size.

And it is desirable to provide an optical filter having a greater degreeof freedom regarding the selection of size in comparison with filterswhich utilize the surface plasmon corresponding to the peripheralstructure in a metal film having a relatively large surface area.

The present invention is directed to an optical filter that transmitslight of a first wavelength, including:

a dielectric substrate;

a first metal structure group in which a plurality of first metalstructures are arranged two-dimensionally in an isolated state in thein-plane direction of the dielectric substrate, that is provided on asurface of the dielectric substrate; and

a dielectric layer with which the first metal structure group iscovered, comprising:

the first metal structures having a first length in a first directionand a second length in a second direction orthogonal to the firstdirection; the first length and the second length being less than orequal to the first wavelength; and

a transmittance of the first wavelength being made minimal or areflectance of the first wavelength being made maximal by localizedsurface plasmons induced on a surface of the first metal structures byresonance between light incident on the dielectric substrate or thedielectric layer and the first metal structures.

In the optical filter, a period at which the first metal structures inthe first metal structure group are arranged can be less than or equalto the first wavelength.

The first length and the second length can be the same.

The first metal structures can be in square shape.

In the optical filter, a thickness of the first metal structures can beless than or equal to the first wavelength.

The first metal structures can consist of aluminium, or an alloy or amixture including aluminium.

In the optical filter, a dielectric constant of the dielectric substrateand a dielectric constant of the dielectric layer can be the same.

The dielectric substrate and the dielectric layer can be comprised ofany one selected from the group consisting of silicon dioxide, titaniumdioxide, and silicon nitride.

The optical filter can comprise a distance from the surface of thedielectric layer to the surface of the first metal structure is lessthan or equal to a value d expressed by the following formula:

$d = \frac{\lambda_{res}^{2}}{2n\;\Delta\;\lambda_{FW}}$in which λ_(res) denotes a plasmon resonance wavelength of the firstmetal structures, n denotes a refractive index of the dielectric layer,and Δλ_(FW) denotes a full width at half maximum of a resonance spectrumof the first metal structure.

In the optical filter, a distance from the surface of the dielectriclayer to the surface of the first metal structures can be less than orequal to a value d expressed by the following formula:

$d = \frac{\lambda_{res} - {\Delta\;\lambda_{HW}}}{{2n}\;}$in which λ_(res) denotes a plasmon resonance wavelength of the firstmetal structures, n denotes a refractive index of the dielectric layer,and Δλ_(HW) denotes a half width at half maximum of a resonance spectrumof the first metal structure.

The optical filter can comprises that the first length and the secondlength are within a range of 110 nm or more and 160 nm or less, athickness of the first metal structures is within a range of 10 nm ormore and 200 nm or less, a period at which the first metal structuresare arranged is within a range of 340 nm or more and 450 nm or less, andthe first wavelength is within a range of 550 nm or more and less than650 nm.

The optical filter can comprise that the first length and the secondlength are within a range of 90 nm or more and less than 130 nm, athickness of the first metal structures is within a range of 10 nm ormore and 200 nm or less, a period at which the first metal structuresare arranged is within a range of 260 nm or more and 340 nm or less, andthe first wavelength is within a range of 450 nm or more and less than550 nm.

The optical filter can comprise that the first length and the secondlength are within a range of 60 nm or more and less than 100 nm, athickness of the first metal structures is within a range of 10 nm ormore and 200 nm or less, a period at which the first metal structuresare arranged is within a range of 180 nm or more and 280 nm or less, andthe first wavelength is within a range of 350 nm or more and less than450 nm.

The optical filter can include two or more of the first metal structuregroups in an in-plane direction of the dielectric substrate; and

periods at which the first metal structures comprising the two or morefirst metal structure groups are arranged are different to each other,and the two or more first metal structure groups are arranged indifferent regions of the dielectric substrate surface.

The optical filter can include, in addition to the first metal structuregroup, a second metal structure group in which a plurality of secondmetal structures are arranged two-dimensionally in an isolated state inan in-plane direction of the dielectric substrate, wherein

the second metal structures have a third length in the first directionand a fourth length in the second direction, and the third length andthe fourth length are less than or equal to a second wavelengthdifferent from the first wavelength;

the third length is different to the first length or the fourth lengthis different to the second length;

the first metal structure group and the second metal structure group arearranged in different regions of the dielectric substrate surface; and

a transmittance of the second wavelength is made minimal or areflectance of the second wavelength is made maximal by localizedsurface plasmons induced on a surface of the second metal structures byresonance between light incident on the dielectric substrate or thedielectric layer and the second metal structures.

The optical filter can include two or more of the first metal structuregroups in an in-plane direction of the dielectric substrate,

comprising the two or more first metal structure groups are arranged inoverlapping regions.

The optical filter can include two or more of the first metal structuregroups in an in-plane direction of the dielectric substrate, comprising:

periods at which the first metal structures comprising the two or morefirst metal structure groups are arranged are different to each other;and

the two or more first metal structure groups are arranged in overlappingregions.

The optical filter can include, in addition to the first metal structuregroup, a second metal structure group in which a plurality of secondmetal structures are arranged two-dimensionally in an isolated state inan in-plane direction of the dielectric substrate, comprising:

the second metal structures have a third length in the first directionand a fourth length in the second direction, and the third length andthe fourth length are less than or equal to a second wavelengthdifferent from the first wavelength;

the third length is different to the first length or the fourth lengthis different to the second length;

the first metal structure group and the second metal structure group arearranged in overlapping regions; and

a transmittance of the second wavelength is made minimal or areflectance of the second wavelength is made maximal by localizedsurface plasmons induced on a surface of the second metal structures byresonance between light incident on the dielectric substrate or thedielectric layer and the second metal structures.

The present invention is directed to an optical filter that transmits orreflects light, including:

a dielectric substrate;

a first metal structure group and a second metal structure groupcomprising a plurality of metal structures arranged in an isolated statein an in-plane direction of the dielectric substrate, that are providedon a surface of the dielectric substrate; and

a dielectric layer with which the first and second metal structuregroups are covered, wherein

the first metal structure group and the second metal structure group arearranged in different regions of the dielectric substrate surface;

first metal structures comprising the first metal structure group arearranged in a first direction, and have a first length in the firstdirection and a second length in a second direction that is orthogonalto the first direction, and the first length and the second length arelengths that are less than or equal to the first wavelength;

second metal structures comprising the second metal structure group arearranged in the first direction, and have a third length in the firstdirection and a fourth length in the second direction, with the thirdlength and the fourth length being lengths that are less than or equalto a second wavelength different from the first wavelength, and with thefirst length and the third length being different or the second lengthand the fourth length being different;

a transmittance of a first wavelength is made minimal or a reflectanceof the first wavelength is made maximal by localized surface plasmonsinduced on a surface of the first metal structures; and

a transmittance of a second wavelength is made minimal or a reflectanceof the second wavelength is made maximal by localized surface plasmonsinduced on a surface of the second metal structures.

In the optical filter, a period at which the first metal structures arearranged and a period at which the second metal structures are arrangedcan be the same.

The present invention is directed to a laminated optical filter, inwhich another dielectric layer is formed on a surface of the dielectriclayer that comprises an optical filter according to claim 1, including:

a third metal structure group in which a plurality of third metalstructures are arranged two-dimensionally in an isolated state in thein-plane direction of the dielectric layer surface, that is providedbetween the dielectric layer surface and the other dielectric layer,wherein

the third metal structures comprised of the third metal structure grouphave a fifth length in the first direction and a sixth length in thesecond direction, and the fifth length and the sixth length are lengthsthat are less than or equal to a third wavelength different from a firstwavelength;

the first length and the fifth length are different or the second lengthand the sixth length are different, or a period at which the third metalstructures are arranged is different to a period at which the firstmetal structures are arranged;

a transmittance of a first wavelength is made minimal or a reflectanceof the first wavelength is made maximal by localized surface plasmonsinduced on a surface of first metal structures; and

a transmittance of the third wavelength is made minimal or a reflectanceof the third wavelength is made maximal by localized surface plasmonsinduced on a surface of the third metal structures.

The present invention is directed to a light-detecting device comprisingthe optical filter.

The present invention is directed to an image capturing devicecomprising the light-detecting device.

The present invention is directed to a camera comprising the imagecapturing device.

The present invention is directed to an optical filter that transmits orreflects light of the visible light region, including:

a dielectric substrate;

a dielectric layer formed on a surface of the dielectric substrate; and

a first metal structure group provided between the dielectric substrateand the dielectric layer in which a plurality of first metal structuresare arranged periodically and two-dimensionally in an isolated state inthe in-plane direction of the dielectric substrate, comprising:

the first metal structures having a first length in a first directionand a second length in a second direction orthogonal to the firstdirection; the first length and the second length being less than orequal to the light of the visible light region; and

a transmittance of the first wavelength in the visible light regionbeing made minimal or a reflectance of the first wavelength in thevisible light region being made maximal by localized surface plasmonsinduced on a surface of the first metal structures by resonance betweenlight incident on the dielectric substrate or the dielectric layer andthe first metal structures.

According to the present invention it is possible to provide ahigh-transmittance optical filter in which light absorption is less thana hole-type metallic thin film filter in a waveband region containing avisible light region.

Further, according to the present invention it is possible to provide anoptical filter having a greater degree of freedom regarding theselection of size in comparison with filters which utilize the surfaceplasmon corresponding to the peripheral structure in a metal film havinga relatively large surface area.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams that illustrate a firstembodiment of the present invention.

FIG. 2 is a diagram illustrating a transmission spectrum obtained by thefirst embodiment.

FIG. 3 is a schematic diagram illustrating a second embodiment.

FIGS. 4A and 4B are schematic diagrams that illustrate a fourthembodiment.

FIG. 5 is a schematic diagram illustrating a fifth embodiment.

FIG. 6 is a schematic diagram illustrating the fifth embodiment.

FIG. 7 is a schematic diagram illustrating a sixth embodiment.

FIGS. 8A, 8B and 8C are schematic diagrams of an optical filteraccording to Example 1.

FIGS. 9A and 9B are diagrams that illustrate transmission spectraobtained by the optical filter according to Example 1.

FIGS. 10A, 10B and 10C are schematic diagrams of an optical filteraccording to Example 2.

FIG. 11 is a diagram that illustrates transmission spectra obtained bythe optical filter according to Example 2.

FIGS. 12A, 12B and 12C are schematic diagrams of an optical filteraccording to Example 3.

FIG. 13 is a diagram that illustrates transmission spectra obtained bythe optical filter according to Example 3.

FIG. 14 is a schematic diagram that is used when describing anembodiment of the present invention.

FIGS. 15A and 15B are schematic diagrams that illustrate an opticalfilter according to Example 4 and transmission spectra.

FIG. 16 is a schematic diagram that illustrates a third embodiment.

FIG. 17 is a view that illustrates the relationship between the lengthof a metal structure and a resonance wavelength.

FIGS. 18A, 18B and 18C are schematic diagrams in which the lengths ofmetal structures are defined.

FIG. 19 is a view that illustrates the relationship between the lengthof a metal structure and a resonance wavelength.

FIG. 20 is a view that illustrates the relationship between thethickness of a metal structure and a resonance wavelength.

FIG. 21 is a view that illustrates the relationship between the periodof metal structures and a resonance wavelength.

FIG. 22 is a view that shows the relationship between the wavelength andthe transmittance.

FIG. 23 is a schematic view of the light-detecting device of the presentinvention.

FIG. 24 is a schematic view of the image pickup device of the presentinvention.

FIG. 25 is a schematic view of the digital camera of the presentinvention.

BEST MODES FOR CARRYING OUT THE INVENTION

The inventors of the present invention focused on the fact that thetransmittance of light is low in a hole-type metallic thin film, andstudied a dot-type optical filter in which a plurality of metalstructures are arranged on a dielectric substrate.

A metal particle, in particular a particle that is approximately thesize of a light wavelength or a size less than that, can generatelocalized surface plasmon resonance (LSPR).

In this case, the term “plasmon” refers to collective oscillation offree electrons on a metal surface that is excited by an externalelectric field such as light. Because electrons are electricallycharged, polarization occurs due to the density distribution of freeelectrons that is caused by oscillation of electrons. A phenomenon inwhich that polarization and an electromagnetic field are combined isreferred to as “plasmon resonance”.

In particular, a resonance phenomenon between light and plasmaoscillations of free electrons produced on a metal microstructure or ametal particle surface is referred to as localized surface plasmonresonance (LSPR).

More specifically, collective oscillation of free electrons on a metalparticle surface is excited by an external electric field such as light,density distribution of electrons and polarization accompanying thatdensity distribution is produced by the oscillation, and anelectromagnetic field arises that localizes in the vicinity of theparticle.

Comparing filters of the same surface area, it is found that the metalportion of a dot-type optical filter in which a plurality of metalstructures are arranged (for example periodically arranged) on adielectric substrate can be decreased in comparison to a hole-typeoptical filter. Therefore, since a dot-type optical filter offers aconfiguration that facilitates a large aperture portion and in whichabsorption of light by metal can be suppressed, the overalltransmittance can be increased in comparison to a hole-type opticalfilter.

FIG. 14 is a schematic diagram of a dot-type optical filter in whichpluralities of metals 1402 are disposed (for example, at a certainperiod) on a dielectric substrate 1401. By adopting this configuration,it is possible to obtain a transmission spectrum having a minimum valueof transmittance at a specific wavelength.

It is due to light of a specific wavelength is absorbed and dispersed bythe LSPR that occurs, a transmittance minimum value arises in thetransmission spectrum.

As long as a metal structure has a material thickness of several nm ormore, the occurrence of LSPR can be caused using even a minute metalstructure.

The inventors of the present invention studied the optical filtersutilizing localized surface plasmon resonance.

However, when the inventors of the present invention progressed thestudy, they found some undesirable phenomena for an optical filter inthe case of the metals 1402 are merely arranged on top of the dielectricsubstrate 1401.

More specifically, when the metals 1402 are merely arranged on top ofthe dielectric substrate 1401, there is a difference between thefrequency of plasmon resonance at the boundary (metal upper surface1403) between air and the metals and the frequency of plasmon resonanceat the boundary (metal lower surface 1404) between the metals 1402 andthe dielectric substrate 1401. It was found that, as a result,enlargement of the optical spectrum width or peak splitting occurs, andcharacteristics that are not preferable for an optical filter appear.

Further, when using this optical filter as a reflective filter, thereflection characteristics differ according to whether incident light isincident from the dielectric substrate side or from the metal side.Therefore, to manifest the desired optical characteristics, the opticalfilter must allow light to be incident from only one given direction,and there is the possibility that the degree of design freedom of anoptical system that uses this kind of optical filter will be reduced.

There is also the problem that a shift in a peak wavelength occurs ifdirt attaches to the surface of the metals.

Therefore, the inventors of the present invention studied structures inwhich metals are embedded in a dielectric material. As a result, theinventors of the present invention succeeded in suppressing splitting ofspectral peaks and peak width enlargement that are caused by adifference in plasmon resonance frequencies at a boundary between airand metal.

Further, the inventors of the present invention found that it is alsopossible to prevent oxidation of metal and to suppress changes (shiftsin peak wavelengths) in optical characteristics caused by dirt adheringto the metal surfaces.

In this connection, when attempting to use a device such as a coloringfilter or a dielectric multilayer filter that is a common opticalfilter, a film thickness that is greater than or equal to the wavelengthof light is necessary. More specifically, a film thickness ofapproximately 1 μm or more is required.

In contrast, according to the optical filter of the present invention,it is possible to construct a filter in which the film thicknessincludes a metal thickness of approximately 100 nm or less. Since theoverall film thickness can be kept to approximately 200 nm even if aprotective layer of approximately 100 nm is laminated on top of themetal structures, it is possible to provide a filter that has a thinfilm thickness in comparison to a conventional filter.

Consequently, if the optical filter according to the present inventionis used in a light receiving element such as a CCD sensor or a CMOSsensor, the light receiving element can be made smaller. Further, if theoptical filter according to the present invention is used in a lightreceiving element, it is also possible to alleviate insufficiencies inthe amount of received light that are caused by a decrease in theobservation angle of each pixel that accompanies the provision of alarge number of pixels in the light receiving element.

First Embodiment Single-Layer Optical Filter and Laminated OpticalFilter

Hereunder, an embodiment of the present invention is described using thedrawings. First, the items denoted by reference numerals in the FIGS.are listed: 110: dielectric substrate, 120: metal structure, 121: firstmetal structure, 122: first metal structure, 130: dielectric layer, 140:first direction, 141: first length, 145: period, 150: second direction,151: second length, 155: period, 160: thickness of metal structure, 170:thickness after subtracting metal structure thickness from dielectriclayer thickness, 201: transmission spectrum R, 202: transmissionspectrum G, 203: transmission spectrum B, 301: region, 302: region, 303:region, 401: first metal structure, 402: first metal structure group,403: first metal structure, 404: first metal structure group, 405:period, 406: period, 407: first metal structure, 408: second metalstructure, 501: first metal structure group, 502: first direction, 503:second direction, 504: first length, 505: second length, 506: secondmetal structure group, 507: third length, 508: fourth length, 701:dielectric substrate, 702: first metal structure group, 703: firstdielectric layer, 704: third metal structure group, 705: seconddielectric layer (other dielectric layer), 801: dielectric substrate,802: metallic thin film layer, 803: resist, 804: metallic thin filmstructure, 805: dielectric layer, 901: transmission spectrum R, 902:transmission spectrum G, 903: transmission spectrum B, 904: reflectionspectrum R, 905: reflection spectrum G, 906: reflection spectrum B,1001: dielectric substrate, 1002: metallic thin film layer, 1003: resistfor electron beam lithography, 1004: pattern portion A, 1005: patternportion B, 1006: pattern portion C, 1007: metallic thin film structure,1008: dielectric layer, 1101: transmission spectrum R, 1102:transmission spectrum G, 1103: transmission spectrum B, 1201: dielectricsubstrate, 1202: first metallic thin film layer, 1203: resist, 1204:first metallic thin film structure, 1205: first dielectric layer, 1206:second metallic thin film layer, 1207: second metallic thin film layer,1208: second dielectric layer, 1301: transmission spectrum, 1302:transmission spectrum, 1303: laminated filter transmission spectrum,1401: dielectric substrate, 1402: metal, 1403: metal upper surface,1404: metal lower surface, 1501: metal structure, 1502: metal structure,1503: transmission spectrum, 1504: transmission spectrum, 1505:transmission spectrum, 1506: period, 1601: first metal structure, 1602:first metal structure group, 1603: second metal structure, 1604: secondmetal structure group, 1801: first length, 1802: second length, 2201:transmission spectrum, 2202: transmission spectrum.

FIG. 1B is a top view of an optical filter that is a first embodiment ofthe present invention. FIG. 1A is a sectional view along a line 1A-1Ashown in FIG. 1B.

A dielectric layer 130 is provided on the surface of a dielectricsubstrate 110. A plurality of metal structures 120 are arranged betweenthe dielectric substrate 110 and the dielectric layer 130.

The metal structures 120 are two-dimensionally and periodically arrangedin an isolated state in the in-plane direction of the dielectricsubstrate 110 to constitute a metal structure group. In this connection,for description purposes, two first metal structures that constitute afirst metal structure group are denoted by reference numerals 121 and122.

The optical filter of the present invention utilizes localized surfaceplasmon resonance induced in a metal structure itself. In the invention,the periodical arrangement of the plurality of the metal structures 120is preferable as described blow.

When the localized surface plasmon is induced in a metal structure, anelectrical field penetrates from the metal structure. So whenpluralities of the metal structures are arranged in the range ofpenetration length of the electrical field, resonance condition of eachmetal structure is effected each other. To decrease the effect it ispreferable in the invention that a metal structure is arranged at aposition where electromagnetic relations between a metal structure andmetal structures adjacent thereto are almost equivalent to each other,which means a periodical arrangement of the metal structures.

The periodical arrangement of metal structures restrains a mismatchingof the plasmon resonance condition between the metal structures, and, alocalized surface plasmon having the same wavelength and the same phasecan be induced in each structure. Thus a dip in a resonance peak in atransmission spectrum can be deeper and a width of the peak can benarrower. And also as a giving of diffracted light can be restrained, aneffect on a shape of transmission spectrum can be reduced.

In the case where a plurality of metal structures is arranged too closeto each other, the resonance conditions of the metal structures arestrongly affected by each other so that there is a fear that a desirableresonance wavelength or spectrum width might not be obtained and atransmittance might be decreased. Considering that an electrical fieldfrom the metal structure penetrates almost up to the same length as themetal structure its self's size when localized plasmon resonance isobtained, it is preferable that a plurality of the metal structures arearranged at a distance of approximately equal to the size of the metalstructure.

Also it is preferable that the metal structures are arranged apart fromeach other to such a degree that the above-mentioned penetration lengthis not overlapped, namely arranged apart at a distance approximatelytwice or more as long as the size of the metal structure.

On the other hand, a distance between each metal structure becomeslarger to such a degree of almost three times as long as the size of themetal structure, a dip in a transmission spectrum becomes shallow.

Accordingly, concerning a distance between each metal structure arrangedperiodically it is preferable that the distance is equal to the size ofthe metal structure its self or more, and between the range up to threetimes the metal structure's size, further approximately twice as long asthat.

In FIG. 1B, the first metal structure 121 includes a first length 141 ina first direction 140, and includes a second length 151 in a seconddirection 150 that is orthogonal to the first direction 140. In thiscase, the first length 141 and the second length 151 are set to, forinstance, a length that is less than or equal to an optical wavelengthin a visible light region. Even when the wavelength of plasmons inducedat the metal structure is in the mode of the lowest order (dipolarmode), the half-wavelength of plasmons is substantially the same as thesize of a metal structure. Thus, because the size of a structure onwhich plasmons can be excited with e.g. visible light is shorter than anexcitation wavelength of visible light, these lengths are made less thanor equal to an optical wavelength in a visible light region.

Further, it is preferable to make the first length 141 and the secondlength 151 less than or equal to the plasmon resonance wavelength (thefirst wavelength).

In this case, as one example, the first metal structure 121 is assumedto be a square shape in which the first length and the second length arethe same and one side is 120 nm. Although a square shape is preferablein the respect of design ease with regard to optical characteristics,metal of a circular shape, an elliptical shape, or another polygonalshape can also be used as a metal structure. For example, a metal with acircular shape is also suitable since it is possible to suppresspolarization dependency and manufacturing precision is also easilymaintained.

When a metal structure that is not a square shape is used, as shown inFIGS. 18A to 18C, the first length and the second length are handled asthe lengths denoted by reference numerals 1801 and 1802, respectively.

The metal structure is not limited in shape to the above but may havevarious shapes. On the other hand, the first or second length may beregarded as a maximum width of the metal structure.

According to the present embodiment, the transmittance of apredetermined wavelength (the first wavelength) in a visible lightregion is made to amount to a minimal value by localized surfaceplasmons that are induced on the surface of a metal structure byresonance between light that is incident on the dielectric substrate ordielectric layer and the metal structure.

For the first metal structure group shown in FIGS. 1A and 1B, a form canalso be adopted in which a period 145 and a period 155 at which themetal structures 120 are arranged may be equal to or less than anoptical wavelength in a visible light region and preferably equal to orless than a plasmon resonance wavelength (the first wavelength). This isbecause, when the periods at which metal structures are arranged aregreater than the optical wavelength region of interest, there is apossibility that diffracted light of a high order will occur and theintensity of zero-order diffracted light will decrease.

Further, a form can also be adopted in which the period 145 and theperiod 155 at which the metal structures 120 are arranged are less thana plasmon resonance wavelength (first wavelength) of the first metalstructure group. This is because, when the periods at which the metalstructures are arranged are near to the plasmon resonance wavelength,light of a wavelength that causes Wood's anomaly combines with theplasmon resonance such that a peak shape caused by the plasmon resonancesharpens and the depth of a transmittance minimum value at the resonancewavelength becomes shallow. In this case, the term “Wood's anomaly”refers to a phenomenon whereby incident light is diffracted by aperiodic structure, and the diffracted light is propagated extremelynear to the metal periodic structure surface in a manner that isparallel with the surface.

In this case, as one example, with the object of generating plasmonresonance in a red wavelength band, the periods 145 and 155 are assumedto be 400 nm.

A form can also be adopted in which a space 152 between the first metalstructures 121 and 122 is greater than the first length 141 and thesecond length 151. By setting this space, it is possible to suppress theenlargement of spectral peak widths or shifts in peak wavelengths thatare caused by near-field interaction between metal body structures.

Further, a form can also be adopted in which a thickness 160 of themetal structures 120 may be less than or equal to an optical wavelengthin a visible light region and preferably equal to or less than a plasmonresonance wavelength (the first wavelength). The reason is that if thethickness of a metal structure is set to be too thick in amicro-fabrication process when producing the filter, a manufacturingerror will be large. In this case, as one example, the thickness 160 isassumed to be 30 nm.

Aluminum, gold, silver, platinum, or the like can be used as thematerial of the metal structures 120. Among these, the plasma frequencyof aluminum is high compared to silver, and with aluminum the design ofa filter with optical characteristics that cover the entire visiblerange is physically easy (Ag: ˜3.8 eV (˜325 nm), Al: ˜15 eV (˜83 nm)).

Further, in comparison to silver and the like, aluminum is less likelyto undergo oxidation and is chemically stable, and can thus stablyexpress predetermined optical characteristics for a long period.

Furthermore, since an imaginary part of the dielectric constant ofaluminum is larger than in the case of silver, aluminum can achieve anadequate light blocking effect in comparison to silver even if the filmthickness is thin, and micromachining is also easy.

In addition, since aluminum is extremely inactive chemically, similarlyto platinum, aluminum also has no drawbacks such as difficulty inmicromachining by dry etching.

In this connection, the metal structures 120 may also be an alloy or amixture including aluminum, gold, silver, and platinum.

The material of the dielectric substrate 110 can be suitably selectedfrom metallic oxides such as titanium dioxide or quartz (silicondioxide) that are materials that transmit light of e.g. a visible lightregion, or materials that have a high transmittance such as siliconnitride. Further, a high polymer material such as polycarbonate orpolyethylene terephthalate can also be used as the material of thedielectric substrate 110.

Similarly to the dielectric substrate 110, the material of thedielectric layer 130 can be suitably selected from quartz (silicondioxide), titanium dioxide, silicon nitride and the like. Further, ahigh polymer material such as polycarbonate or polyethyleneterephthalate can also be used as the material of the dielectric layer130.

A difference between the dielectric constant of the dielectric substrate110 and that of the dielectric layer 130 can be 5% or less. The reasonis that, when there is a large difference between the dielectricconstant of the dielectric substrate 110 and the dielectric constant ofthe dielectric layer 130, there is a large difference between theexcitation wavelength of plasmons occurring at the boundary of the metalstructures 120 and the dielectric substrate 110 and the excitationwavelength of plasmons occurring at the boundary of the metal structures120 and the dielectric layer 130. As a result, there is a risk that apeak of a resonance wavelength other than a desired peak will occur orthat peak width enlargement will occur.

Therefore, it is most preferable that the dielectric constant of thedielectric substrate and the dielectric constant of the dielectric layerare identical.

A thickness 170 that is obtained by subtracting the thickness 160 of themetal structures 120 from the thickness of the dielectric layer need notbe thick. The reason is that, if the thickness of the dielectric layeris too thick, because the dielectric layer 130 forms one kind ofFabry-Perot resonator, there is a concern that a large number of minutedips will appear in the transmission spectrum.

Therefore, it is suitable that, for example, the resonator modes of aFabry-Perot resonator do not exist within a wavelength range of fullwidth at half maximum of the plasmon resonance of the metal structures120.

To achieve this, it is necessary that at least a range (FSR) of theresonator modes is wider than the full width at half maximum of theplasmon resonance.

This condition is expressed below:

$d = \frac{\lambda_{res}^{2}}{2n\;\Delta\;\lambda_{FW}}$where λ_(res) denotes a plasmon resonance wavelength of the metalstructures, n denotes a refractive index of the dielectric layer, andΔλ_(FW) denotes a full width at half maximum of the resonance spectrumof the metal structures.

In this case, the symbol d in the equation corresponds to the thicknessof the dielectric layer. Accordingly, since, for example, the full widthat half maximum of the plasmon resonance is typically 100 nm, if theplasmon resonance wavelength is assumed to be 650 nm and the refractiveindex of the dielectric layer is assumed to be 1.46, d is calculated tobe 1447 nm. For this reason, when the wavelength region of interest is650 nm±50 nm, to ensure that the FSR is 100 nm or more in thiswavelength region it is necessary that the thickness of the dielectriclayer is equal to or less than this value d.

A configuration is also preferable embodiment to ensure that theresonator modes of the Fabry-Perot resonator only appear in a wavelengthregion that is shorter than the wavelength region of interest.

The resonator modes of the Fabry-Perot resonator occur at a wavelengththat is equal to two times the resonator length. When the wavelengthregion of interest is assumed to be a wavelength region that is insidethe resonance width of the metal structures, the shortest wavelength ofthe wavelength region of interest is a value obtained by subtracting thehalf width at half maximum of the resonance from the resonancewavelength. Therefore, in order for the resonator modes of theFabry-Perot resonator to be shorter than this wavelength, it isnecessary to make the thickness of the dielectric layer less than orequal to a value d indicated by the following equation:

$d = \frac{\lambda_{res} - {\Delta\;\lambda_{HW}}}{{2n}\;}$where λ_(res) denotes a plasmon resonance wavelength of the metalstructures, n denotes a refractive index of the dielectric layer, andΔλ_(HW) denotes a half width at half maximum of the resonance spectrumof the metal structures.

For example, when it is assumed that the resonance wavelength is 450 nm,the half width at half maximum of the resonance is 50 nm, and therefractive index is 1.46, d is calculated to be 137 nm. Therefore, toensure that the Fabry-Perot resonator modes do not appear on the side ofa wavelength shorter than 400 nm that is the shortest wavelength in thevisible light region, the thickness of the dielectric layer can be madeless than or equal to this value d.

In contrast, it is unsuitable for the dielectric layer thickness to betoo thin, and it is favorable for the dielectric layer to have a certaindegree of thickness. That is, it is suitable for the thickness 170obtained by subtracting the thickness of the metal structures 120 fromthe thickness of the dielectric layer to be greater than or equal to thefirst length 141 or the second length 151 of the metal structures 120.Further, the thickness 170 can be at least around 100 nm.

This is because the spread of a near field that is generated by themetal structures 120 is typically around the size of the metalstructures 120 itself or around 100 nm. If a space within a distancethat is about the extent of a near-field region produced by the metalstructures 120 from the metal structures 120 surface is occupied by adielectric layer, it is possible to suppress the occurrence of a statein which foreign matter in mixed in the near-field region produced bythe metal structures 120 and the optical characteristics of the metalstructures 120 change.

(Calculation Results)

FIG. 2 is a view that illustrates results obtained by performingnumerical calculations using the above described structure. Morespecifically, FIG. 2 illustrates results obtained using an opticalfilter that uses aluminum for metal structures that have a first lengthand a second length of 120 nm, are arranged at a period of 400 nm, andhave a thickness of 30 nm. The transmission spectrum of this opticalfilter is illustrated by a transmission spectrum 201, and it is foundthat the filter functions as an optical filter that strongly absorbslight in the vicinity of a 650-nm wavelength.

Because the 650-nm wavelength is a band for red, the first letter of theword “Red” is used and the filter is referred to as “optical filter R”.Since this optical filter reflects and absorbs wavelengths of red, thecolor cyan that is a complementary color of red can be observed as atransmission spectrum.

It is also possible to vary the wavelength, spectral bandwidth, andintensity of the transmission spectrum by varying the diameter or periodof the metal structures.

For example, by making the length 100 nm, the period 310 nm, and thethickness 30 nm, an optical filter can be constructed that has atransmission spectrum 202 that has absorbance in the vicinity of green(wavelength 550 nm) in the visible range. This is referred to as“optical filter G”. When the transmission spectrum of optical filter Gis observed, the color magenta which is a complementary color of greencan be observed.

Likewise, by making the length 70 nm, the period 250 nm, and thethickness 30 nm, an optical filter can be constructed that has atransmission spectrum 203 that has absorbance in the vicinity of blue(wavelength 450 nm) in the visible range. This is referred to as“optical filter B”. When the transmission spectrum of filter B isobserved, the color yellow which is a complementary color of blue can beobserved.

In this connection, with respect to a reflection spectrum of the opticalfilter according to the present embodiment, the reflectance amounts to amaximal value in the vicinity of a wavelength at which the transmittanceamounts to a minimal value. Therefore, the optical filter according tothe present embodiment can also be used as a reflective filter, and notonly as a transmission filter.

(Design Guidelines)

Hereunder, the relation between optical characteristics and parametersfor configuring a metal structure group are described.

Localized surface plasmon resonance that is induced at a metal structureis an electric charge density distribution that accompanies plasmaoscillations of free electrons inside the metal structure. The electriccharge density distribution or optical characteristics of the metalstructure are influenced by the shape of the structure.

For example, if the length of the metal structures in the direction ofpolarization of light irradiated on the metal structures is increasedwhile keeping the length of the metal structures in a directionorthogonal to the direction of polarization, the thickness of the metalstructures, and the period at which the metal structures are arrangedconstant, the resonance wavelength shifts to a long wavelength side.

It is thus found that in order to generate a wavelength of localizedsurface plasmon resonance of the metal structures on the long wavelengthside, it is sufficient to increase the length in the direction ofpolarization of the metal structures. This tendency is illustrated inFIG. 17. Table 1 below illustrates the correspondence between therelation illustrated in FIG. 17 and peak width and transmittance.

TABLE 1 Length in direction of Resonance polarization wavelength Peakwidth Transmittance 150 760 100 40 200 870 150 15 250 980 250 10 3001100 410 5

Length in orthogonal direction to polarization fixed at 100 nm,thickness fixed at 50 nm, and period fixed at 500 nm

According to Table 1, it is found that as the length in the direction ofpolarization of metal particles increases, not only does the resonancewavelength shift to a long wavelength, but the peak width broadens andthe transmittance at the absorption peak decreases. In this connection,it is not always necessary that polarization of light incident on theoptical filter is strictly along the direction of the length or thedirection of the width of the metal particle.

Further, as shown in FIG. 19, as the length of metal structures in theorthogonal direction to polarization increases, the resonance wavelengthshifts to a short wavelength side. Table 2 below illustrates thecorrespondence between the relation illustrated in FIG. 19 and peakwidth and transmittance.

TABLE 2 Length in orthogonal direction to Resonance polarizationwavelength Peak width Transmittance 100 1100 410 5 150 1040 450 4 200980 515 3 250 950 545 2 300 910 550 2

Length in direction of polarization fixed at 300 nm, thickness fixed at50 nm, and period fixed at 500 nm

According to Table 2, it is found that as the length of the metalstructures in the orthogonal direction to polarization increases, theresonance width broadens and the transmittance at the resonancewavelength tends to decrease.

FIG. 20 illustrates the relation between resonance wavelength andthickness. Further, Table 3 below illustrates the correspondence betweenthe relation illustrated in FIG. 20 and peak width and transmittance.

TABLE 3 Resonance Thickness wavelength Peak width Transmittance 50 1100410 5 100 1010 370 4 300 777 285 1

Length in direction of polarization fixed at 300 nm, length inorthogonal direction to polarization fixed at 100 nm, and period fixedat 500 nm

As shown in FIG. 20 and Table 3, it is found that when the thickness ofthe metal structures increases, the resonance wavelength becomesshorter, the transmittance at the resonance wavelength decreases, andthe resonance width tends to decrease to some extent.

Using these facts, as shown in FIG. 22, it is possible to improve thespectrum shape from the transmission spectrum 2201 to the transmissionspectrum 2202.

In the transmission spectrum 2201, a sharp dip due to Wood's anomaly ispresent in the spectrum in the vicinity of wavelength 530 nm. Thetransmission spectrum 2201 is an optical spectrum in a case where squaremetallic dots with sides measuring 150 nm in length comprising aluminumof a film thickness of 90 nm are arranged at periods of 400 nm in anequilateral triangular lattice shape. On the other hand, thetransmission spectrum 2202 is an optical spectrum in a case in which thefilm thickness is increased to 150 nm.

More specifically, by shifting a resonance wavelength of a dot array tothe short wavelength side and overlapping with a sharp dip of Wood'sanomaly as a result of increasing the film thickness, the spectrum shapecan be made to have a single peak and the resonance width can be made tocomprise a narrower band.

By making the metal film thickness a predetermined value in this manner,it is also possible to hide a dip that is not preferable in terms of thespectrum.

FIG. 21 illustrates the relation between resonance wavelength andperiod. Further, Table 4 below illustrates the correspondence betweenthe relation illustrated in FIG. 21 and peak width and transmittance.

TABLE 4 Arrangement Resonance period wavelength Peak width Transmittance400 1070 650 2 500 1100 410 5 700 1210 200 15

Length in direction of polarization fixed at 300 nm, length inorthogonal direction to polarization fixed at 100 nm, and thicknessfixed at 50 nm

As shown in FIG. 21 and Table 4, it is found that when the period atwhich metal structures are arranged increases, the resonance wavelengthbecomes longer, the transmittance at the resonance wavelength increases,and the resonance width tends to decrease.

Based on these findings, it is possible to attempt to optimize theparameters for metal structures and a metal structure group, and it isalso possible to design an optical filter having a resonance wavelengthat a desired wavelength.

According to the studies of the inventors of the present invention, inorder to make the resonance wavelength of an optical filter that of thered band (550 nm or more to less than 650 nm), it is necessary to setthe first length and the second length of the metal structures in arange from 110 nm or more to 160 nm or less. Further, it is necessary toset the thickness of the metal structures in a range from 10 nm or moreto 200 nm or less, and to set the period in a range from 340 nm or moreto 450 nm or less.

Further, in order to make the resonance wavelength of the optical filterthat of the green band (450 nm or more to less than 550 nm), it isnecessary to set the first length and the second length of the metalstructures in a range from 90 nm or more to less than 130 nm. Further,it is necessary to set the thickness of the metal structures in a rangefrom 10 nm or more to 200 nm or less, and to set the period at which themetal structures are arranged in a range from 260 nm to 340 nm, andpreferably a range from 270 nm to 330 nm.

Furthermore, in order to make the resonance wavelength of the opticalfilter that of the blue band (350 nm or more to less than 450 nm), it isnecessary to set the first length and the second length of the metalstructures in a range from 60 nm or more to less than 100 nm. Further,it is necessary to set the thickness of the metal structures in a rangefrom 10 nm or more to 200 nm or less, and to set the period at which themetal structures are arranged in a range from 180 nm or more to 280 nmor less, and preferably a range from 200 nm to 270 nm.

The optical filter of the present embodiment thus has an absorbance orreflectance peak of wavelength in the visible light region, as explainedabove. On the other hand, the reducing of the size of the metalstructure and the periphery from the designed values regarding thevisible light region can realize the optical filter for near-ultravioletregion. Also the increasing of the size of the metal structure and theperiphery can realize the optical filter for near-infrared region.

Second Embodiment Bayer Array

According to the present embodiment, an RGB filter arranged in a Bayerarray is described.

In FIG. 3, for example, the aforementioned optical filter R(transmission spectrum 201) is arranged in a region 301, the opticalfilter G (transmission spectrum 202) is arranged in a region 302, andthe optical filter B (transmission spectrum 203) is arranged in a region303. By arranging the optical filters in this manner, it is possible toconstruct a color filter that is arranged in a Bayer array using thefilters according to the present invention.

The above color filter can be used as a color filter for image capturingdevice. Region 301 etc. of the color filter has an area corresponding toone pixel, which may be greater than an area covered by thephotoelectric conversion device (photoelectric conversion part).

According to the present embodiment, the sizes of the metal structuresand the periods at which the metal structures are arranged differ foreach region. However, the present embodiment is not limited to thisconfiguration. For example, metal structure groups with respect to whichonly the periods at which the metal structures are arranged aredifferent may be arranged in each region. Further, metal structuregroups with respect to which only the sizes of the metal structures aredifferent may be arranged in each region.

That is, a configuration may be adopted which includes two or more firstmetal structure groups, in which periods at which the first metalstructures are arranged are different, and in which the first metalstructure groups are arranged in different regions of the dielectricsubstrate surface.

Further, a second metal structure group comprising second metalstructures that are a different shape to the first metal structures thatconstitute the first metal structure group may be arranged in eachregion. More specifically, the second metal structures have a thirdlength in a first direction and a fourth length in a second direction,and the third length is different to the first length of the first metalstructures or the fourth length is different to the second length of thefirst metal structures. The third and fourth lengths are preferablyequal to or less than the second wavelength.

As a result, the second metal structure group can decrease thetransmittance of light at a wavelength (second wavelength) that isdifferent to the resonance wavelength (first wavelength) of the firststructure group.

In the present specification, the terms “first metal structure group”and “second metal structure group” refer to the fact that the shape ofmetal structures constituting the respective structure groups aredifferent. More specifically, even if the periods at which metalstructures are arranged are different, as long as the shapes of themetal structures are the same the term “first metal structure group” isused. Further, if the shape of the metal structures is different, theterm “second metal structure group” is used regardless of whether theperiods are the same or different.

Third Embodiment Triangular Lattice

FIG. 16 is a view that illustrates an embodiment in which metalstructures are arranged in a triangular lattice shape. In the case of atriangular lattice arrangement, since the unit vector components of thelattice are not orthogonal, it is possible to reduce the dependence withrespect to incident light polarization of the optical characteristics ofthe filter in comparison to a tetragonal lattice shape arrangement.

This kind of triangular lattice arrangement can also be expressed as aplurality of metal structure group arranged in a tetragonal latticeshape being disposed in overlapping regions.

More specifically, it is possible to express this arrangement in termsof a first metal structure group 1602 including first metal structures1601 and a second metal structure group 1604 including second metalstructures 1603 being arranged in overlapping regions.

Fourth Embodiment Overlapping of Two or More Structure Groups

According to the present embodiment, similarly to the third embodiment,an example is described in which a plurality of metal structure groupsare overlappingly arranged.

FIG. 4A is a view that illustrates a case in which first metal structuregroups with different periods are overlappingly arranged. First metalstructures 401 that constitute a first metal structure group 402 arearranged at a period 405, while first metal structures 403 thatconstitute a first metal structure group 404 are arranged at a period406. Thus, according to the present embodiment, because the arrangementperiods of the metal structures are different to each other, it ispossible to simultaneously manifest the respective opticalcharacteristics of the two metal structure groups.

That is, the optical filter illustrated in FIG. 4A includes two or moreof the aforementioned first metal structure groups in the in-planedirection of the dielectric substrate, and periods at first structuresthat constitute the two or more first metal structure groups arearranged are mutually different. Further, the two or more first metalstructure groups are arranged in overlapping regions.

FIG. 4B is a view that illustrates a case in which first metalstructures and second metal structures are arranged in overlappingregions. First metal structures 407 constitute a first metal structuregroup, and second metal structures 408 constitute a second metalstructure group. Thus, because the shapes of the metal structuresconstituting the metal structure groups are different, it is possible tosimultaneously manifest the respective optical characteristics of thetwo metal structure groups.

More specifically, in addition to the first metal structure group, theoptical filter illustrated in FIG. 4B includes a second metal structuregroup in which a plurality of second metal structures aretwo-dimensionally and periodically in an isolated state in the in-planedirection of the dielectric substrate. The second metal structures havea third length in a first direction and a fourth length in a seconddirection, and the third length and the fourth length are less than orequal to the second wavelength different from the first wavelength. Thethird length is different to the first length, or the fourth length isdifferent to the second length, and the first metal structure group andthe second metal structure group are arranged in overlapping regions. Asa result, a resonance wavelength (first wavelength) of the first metalstructures and a resonance wavelength (second wavelength) of the secondmetal structures are different.

Fifth Embodiment Single-Row Filter

The present embodiment describes a single-row filter.

In FIG. 5, first metal structures 509 have a first length 504 in a firstdirection 502, and have a second length 505 in a second direction 503that is an orthogonal direction to the first direction 502. The firstmetal structures 509 are periodically arranged in the first direction502 to thereby constitute a first metal structure group 501.

Further, second metal structures 510 have a third length 507 in thefirst direction 502, and have a fourth length 508 in the seconddirection 503. The second metal structures 510 are periodically arrangedin the first direction 502 to thereby constitute a second metalstructure group 506.

The metal structure groups 501 and 506 generate plasmon resonance withrespect to light of respectively different wavelengths. It is thereforepossible to decrease the transmittance of light of differentwavelengths. Therefore, with respect to the optical filter having thestructure shown in FIG. 5, since wavelengths at which the transmittancedecreases vary depending on the irradiation position of light, thefilter can be used for light dispersion and the like.

In this connection, although in the configuration shown in FIG. 5 theperiod of the metal structures 509 in the first direction is differentto the period of the metal structures 510 in the first direction, asshown in FIG. 6, the periods may be the same.

That is, the optical filter according to the present embodiment includesa first metal structure group and a second metal structure group whichhave a plurality of metal structures that are arranged in an isolatedstate in the in-plane direction of the dielectric substrate. The firstmetal structure group and the second metal structure group are arrangedin different regions on the dielectric substrate surface. The firstmetal structures are periodically arranged in the first direction. Thefirst length and the second length of the first metal structures are alength that is less than or equal to the first wavelength. Further, thesecond metal structures constituting the second metal structure groupare periodically arranged in the first direction. The second metalstructures have a third length in the first direction and have a fourthlength in the second direction. The third length and the fourth lengthare lengths that are less than or equal to the second wavelengthdifferent from the first wavelength. The first length and the thirdlength are different, or the second length and the fourth length aredifferent. As a result, a resonance wavelength (first wavelength) of thefirst metal structures is different to a resonance wavelength (secondwavelength) of the second metal structures.

Sixth Embodiment Laminated Optical Filter

The present embodiment describes a laminated optical filter.

In FIG. 7, a first metal structure group 702 is formed on a dielectricsubstrate 701, and this is covered with a first dielectric layer 703.Further, a third metal structure group 704 is arranged on the firstdielectric layer 703, and a second dielectric layer (other dielectriclayer) 705 is formed thereon.

Thereby, it is also possible to make an optical filter with atransmission spectrum that is represented by the product of thetransmission spectrum 201 and the transmission spectrum 203 that areillustrated in FIG. 2. For example, by laminating filters of the opticalfilter R and the optical filter B, an optical filter can be made thathas a transmission spectrum of the product of the transmittance of theoptical filter R and the optical filter B. In this filter, the maximumvalue of transmittance arises in the vicinity of the 550-nm wavelength.Thus, by laminating filters that function as filters of complementarycolors when in a single layer, it is also possible to make the filtersfunction as filters of primary colors.

The configurations of the first metal structure group 702 and the thirdmetal structure group 704 differ with respect to the period of in-planearrangement or the shape of the metal structures.

Because the configurations of the first metal structure group 702 andthe third metal structure group 704 are different, each of these layersgenerate plasmon resonance at mutually different wavelengths. As aresult, in the optical filter of the present embodiment, thetransmittance with respect to at least two wavelengths is minimal.

That is, this fact means that the optical filter of the presentembodiment includes a function as a bandpass filter allows the passageof wavelengths between the aforementioned two minimal wavelengths.

Accordingly, although the first metal structure group 702 and the thirdmetal structure group 704 that are a single layer, respectively, have afunction as a complementary color filter, by adopting a configuration inwhich these two metal structure groups are laminated, it is possible tomanifest a function of a primary color filter that combines thecharacteristics of both metal structure groups.

More specifically, the laminated optical filter according to the presentembodiment is a laminated optical filter in which another dielectriclayer is formed on a dielectric layer surface. Further, the laminatedoptical filter includes between the dielectric layer surface and theother dielectric layer, a third metal structure group in which aplurality of third metal structures are two-dimensionally andperiodically arranged in an isolated state in an in-plane direction ofthe dielectric layer surface. A third metal structure comprising thethird metal structure group includes a fifth length in a first directionand a sixth length in a second direction, and the fifth length and thesixth length are lengths that are less than or equal to the thirdwavelength different from the first wavelength. The first length and thefifth length are different or the second length and the sixth length aredifferent, or a period at which the third metal structures are arrangedand a period at which the first metal structures are arranged aredifferent. Therefore, the third metal structure group can decreasetransmittance at a resonance wavelength (third wavelength) that isdifferent to a resonance wavelength (first wavelength) of the firststructure group.

In this connection, for the laminated optical filter of the presentembodiment, a form can be adopted in which dielectric layers arelaminated at a lamination interval at which near-field interactionalmost does not arise. More specifically, the lamination interval can be100 nm or more.

Example 1

In Example 1, a method of producing absorbance/reflective filters forred, green, and blue (RGB) and the optical characteristics thereof aredescribed.

FIG. 8A is a view that illustrates a structure formed by depositingaluminium of a thickness of 30 nm as a metallic thin film layer 802 onthe surface of a dielectric substrate 801 comprising a quartz substrateof a thickness of 525 μm, and then coating a resist for electron beam(EB) lithography 803 thereon. The method of forming the metallic thinfilm layer 802 is not limited to deposition, and may be sputtering orthe like.

Next, the resist 803 is subjected to patterning using an EB lithographyapparatus. The resist pattern is formed in a shape in which squares withsides of approximately 120 nm are arranged in a tetragonal lattice shapeat a period of approximately 400 nm. By performing dry etching withplasma comprising a gaseous mixture of chlorine and oxygen using thisresist pattern as an etching mask, metallic thin film structures 804 canbe formed. The dry etching gas is not limited to chlorine and oxygen,and may be argon or another gas.

A method of preparing an etching mask is not limited to EB lithography,and may be photolithography or the like. Further, a method of patterningthe metallic thin film layer 802 may be one in which a resist pattern isformed by EB lithography or photolithography on the dielectric substrate801, and a lift-off process is then performed after forming the metallicthin film layer 802. In addition, the metallic thin film layer 802 maybe formed by direct processing using a focused ion beam processingapparatus (FIB processing apparatus).

Next, a quartz thin film with a thickness of 300 nm is formed as adielectric layer 805 on the metallic thin film structures 804 bysputtering. The optical filter formed in this manner is illustrated inFIG. 8B. In this connection, the method of forming the film is notlimited to sputtering, and film formation may be performed by CVD or byapplying SOG or the like.

FIG. 9A illustrates transmission spectra of an optical filtermanufactured in this manner. A transmission spectrum R is determined asdenoted by reference numeral 901 by numerical calculation, and it isfound that the present filter has a minimum value (absorption peak) oftransmittance in the vicinity of the 650-nm wavelength. Since thewavelength showing an absorption peak corresponds to red in the visiblerange, it is found that the present filter functions as a complementarycolor filter that absorbs red.

Further, by making the diameter of the metallic thin film structures 804approximately 100 nm, the thickness approximately 30 nm, and the periodat which the metallic thin film structures 804 are arrangedapproximately 310 nm, a transmission spectrum G denoted by referencenumeral 902 is obtained. Likewise, by making the diameter of themetallic thin film structures 804 approximately 70 nm, the thicknessapproximately 30 nm, and the period at which the metallic thin filmstructures 804 are arranged approximately 250 nm, a transmissionspectrum B denoted by reference numeral 903 is obtained. These areoptical filters that absorb RGB, and function as complementary colorfilters.

Further, with regard to a reflection spectrum of a filter of the presentexample, the reflectance is greatest at a wavelength that issubstantially the same as the wavelength at which transmittance isminimal.

Therefore, as shown in FIG. 9B, by using an optical filter of thepresent example as a reflective filter, a reflection spectrum R that isdenoted by reference numeral 904 can be obtained from the filter thathas the transmission spectrum R. Likewise, a reflection spectrum G thatis denoted by reference numeral 905 can be obtained from the filter thathas the transmission spectrum G, and a reflection spectrum B that isdenoted by reference numeral 906 can be obtained from the filter thathas the transmission spectrum B. Thus, these optical filters can be madeto function as optical filters that strongly reflect red, green, andblue of the visible range, respectively.

Although the present example was described using an example in whichmetal structures are arranged in a tetragonal lattice shape, the metalstructures may be arranged in a triangular lattice arrangement.

Further, the thickness of the dielectric layer 805 is not limited to 300nm, and the dielectric layer 805 may be thinner than 300 nm. To enablethe width of a near-field region generated by the metal structures to becovered by the dielectric layer, it is suitable that the thickness isabout 100 nm or more.

Example 2

In Example 2, a method of producing an RGB filter arranged in a Bayerarray and the optical characteristics thereof are described. FIG. 10A isa view that illustrates a structure formed by depositing aluminium of athickness of 20 nm as a metallic thin film layer 1002 on the surface ofa dielectric substrate 1001 comprising a quartz substrate with athickness of 525 μm, and then coating a resist 1003 thereon.

Next, the resist 1003 is subjected to patterning using an EB lithographyapparatus. Regarding the shape of the resist pattern, a portion in whichshapes formed by arranging squares having sides of approximately 130 nmin a tetragonal lattice shape at a period of approximately 380 nm arepatterned at an approximately 10 μm angle is taken to be a patternportion A 1004. Further, a shape formed by arranging squares havingsides of approximately 110 nm in a tetragonal lattice shape at a periodof approximately 280 nm is taken to be a pattern portion B 1005.Furthermore, a shape formed by arranging squares having sides ofapproximately 80 nm in a tetragonal lattice shape at a period ofapproximately 200 nm is taken to be a pattern portion C 1006. Astructure is prepared in which each of these pattern portions arearranged as shown in FIG. 10B with a clearance of 10 μm left between therespective pattern portions. Metallic thin film structures 1007 areprepared by dry etching with plasma comprising a gaseous mixture ofchlorine and oxygen using this resist pattern as an etching mask.

Next, a quartz thin film with a thickness of 500 nm is formed as adielectric layer 1008 on the metallic thin film structures 1007 bysputtering. An optical filter formed in this manner is shown in FIG.10C.

In this connection, a light-shielding layer may be formed in the regionsbetween the above described pattern portions to prevent color mixing.Further, when the thickness of the metal structures comprising eachpattern portion is made the same, as in the present example, it ispossible to manufacture each pattern portion within the same process andit is also possible to eliminate boundary lines between the patternportions.

As shown in FIG. 11, the pattern portions A, B, and C prepared in thismanner have a transmission spectrum R denoted by reference numeral 1101,a transmission spectrum G denoted by reference numeral 1102, and atransmission spectrum B denoted by reference numeral 1103. These canfunction as complementary color filters with respect to red, green, andblue, respectively. Further, by making the thickness the same for allthe pattern portions as in the present example, complementary colorfilters for red, green and blue can be manufactured in the same batch.

The optical filter using the metal structure of the present example hasa constitution which enables a plurality of the optical filter differentin absorbed wavelength or reflected wavelength to be simultaneouslyfabricated by merely changing the size of structure or periphery of thearrangement, even if they are the same in thickness.

Generally speaking, it is necessary for fabricating a colorant filterarray, which is a general optical filter, to apply kinds of colorantsseparately through their respective processes. On the other hand, theconstitution of the optical filter of the present example enables theoptical filters different in wavelength to be fabricated through thesame process, whereby the cost of the fabrication can be reduced. Thethickness of the dielectric layer 1008 is not limited to 500 nm. Forexample, to ensure that a FSR of 100 nm or more can be secured in theblue wavelength region (450-nm wavelength), it is suitable that thethickness of the dielectric layer is approximately 690 nm or less whenthe refractive index thereof is 1.46. Further, to enable the width of anear-field region generated by the metal structures to be covered by thedielectric layer, it is also suitable that the thickness is about 100 nmor more.

Example 3

In Example 3, a method of manufacturing a laminated filter and theoptical characteristics thereof are described.

FIG. 12A is a view that illustrates a structure formed by depositingaluminium of a thickness of 30 nm as a first metallic thin film layer1202 on the surface of a dielectric substrate 1201 comprising a quartzsubstrate with a thickness of 1 mm, and then coating a resist forelectron beam (EB) lithography 1203 thereon.

Next, the resist 1203 is subjected to patterning using an EB lithographyapparatus. The resist pattern is formed in a shape in which squares withsides of approximately 120 nm are arranged in a tetragonal lattice shapeat a period of approximately 400 nm. By performing dry etching withplasma comprising a gaseous mixture of chlorine and oxygen using thisresist pattern as an etching mask, first metallic thin film structures1204 are produced.

Next, a quartz thin film with a thickness of 300 nm is formed as a firstdielectric layer 1205 on the first metallic thin film structures 1204 bysputtering. Although the thickness of the first dielectric layer 1205 isnot limited to 300 nm, an inter-layer distance can be secured that doesnot produce a near-field interaction with a second metallic thin filmstructure layer that is manufactured in the next process.

Next, as shown in FIG. 12B, aluminium of a thickness of 30 nm isdeposited as a second metallic thin film layer 1206 on the surface ofthe first dielectric layer 1205. A resist for electron beam (EB)lithography is coated as a resist layer on the second metallic thin filmlayer 1206. Subsequently, patterning of the resist layer is performedwith an EB lithography apparatus. The resist pattern is formed in ashape in which squares with sides of approximately 70 nm are arranged ina tetragonal lattice shape at a period of approximately 250 nm. Byperforming dry etching with plasma comprising a gaseous mixture ofchlorine and oxygen using this resist pattern as an etching mask, secondmetallic thin film structures 1207 are produced.

Next, as shown in FIG. 12C, a quartz thin film with a thickness of 400nm is formed as a second dielectric layer 1208 on the second metalstructures 1207 by sputtering.

FIG. 13 illustrates transmission spectra of the laminated optical filterproduced in this manner. A transmission spectrum 1301 of the firstmetallic thin film structures of this filter has an absorption peak inthe vicinity of a wavelength of approximately 650 nm, and a transmissionspectrum 1302 of the second metallic thin film structures has anabsorption peak in the vicinity of a wavelength of approximately 450 nm.Thus, a laminated filter transmission spectrum 1303 of the filteraccording to the present example has a shape that is the product of thetransmission spectrum 1301 and transmission spectrum 1302. Accordingly,it is found that the laminated filter according to the present examplefunctions as an optical filter that allows green to pass through. Thatis, by forming a structure in which filters that function ascomplementary color filters as single layers are laminated, the filterscan be made to function as primary colors filters.

Example 4

In Example 4, an example is described in which, by alternately arrangingmetal structures of differing sizes, that is, by overlappingly arrangingtwo metal structure groups, a filter having a transmission spectrum inwhich a plurality of transmission spectra are combined can be realizedwith a single layer.

FIG. 15A is a view that illustrates an example in which metal structures1501 comprising aluminium formed in squares with sides of 90 nm andmetal structures 1502 comprising aluminium formed in squares with sidesof 150 nm are arranged. The thickness of these metal structures is 60nm. The metal structures 1501 and 1502 are arranged in mutuallydifferent tetragonal lattice shapes at a period 1506. In this case, theperiod 1506 is 250 nm.

FIG. 15B is a view that illustrates a transmission spectrum 1503 of theoptical filter produced in this manner. Meanwhile, as a reference, atransmission spectrum 1504 of an optical filter in which aluminiumsquares with sides of 90 nm and a thickness of 60 nm are arranged in atetragonal lattice shape at a period of 250 nm is also shown. Further, atransmission spectrum 1505 of an optical filter in which aluminiumsquares with sides of 150 nm and a thickness of 60 nm are arranged in atetragonal lattice shape at a period of 400 nm is shown.

Thus, since the optical filter according to the present example has thetransmission spectrum 1503, it is possible to obtain a spectrum thatmanifests both the characteristics of a two-layer filter while keeping asingle-layer structure.

Further, it can be considered that the transmission spectrum 1503 hasthe characteristic of a filter having a maximum transmittance value inthe vicinity of a 600-nm wavelength. Thus, it is possible to manifestthe functions of two layers of a complementary color filter with asingle layer. In this connection, it is possible to produce the opticalfilter according to the present example more easily in comparison to acase of laminating filters with a single layer of metal structures.

Although a case has been described according to the present example inwhich shapes of two kinds of metal structures are mixed in the sameplane, the shapes of metal particles that exist in the same plane may beof three kinds or more. Further, a configuration may be designed so asto obtain desired optical characteristics by arranging structure groupsat periods that are modulated.

Example 5

The present example is directed to a light-detecting device using theoptical filters explained in Examples 1 through 4, and, an image pickupdevice comprised of an array of the optical detecting devices, whichpickup device is incorporated into a camera.

FIG. 23 is a schematic view of the light-detecting device using theoptical filter of the present invention. A light-detecting device 2507introduces a light incident from the outside through a microlens 2501into a photoelectric converting portion 2505. The photoelectricconverting portion generates a charge according to the incident light.The light-detecting device includes an optical filter 2502 disclosed inthe present application, a dielectric layer 2503, electron circuits 2504and a semiconductor substrate 2506, besides the photoelectric convertingportion. The optical filter 2502 includes such a structure capable ofinducing a plasmon resonance to the light as the metal structure 120 inFIGS. 1A and 1B.

FIG. 24 is a schematic view of the image pickup device using the opticalfilter of the present invention. A pixel area 2600 has a 3×3two-dimensional matrix of the aforementioned light-detecting device (;pixels) 2601 a through 2603 c. Alternatively, e.g. 7680×4320 matrix canbe also used as the matrix of the image area 2600 in FIG. 24.

The vertical scanning circuit 2605 and the horizontal scanning circuit2604 in FIG. 24 are circuits for selecting light-detecting device (;pixel) to be read out among the whole light-detecting devices located inthe pixel area 2600.

FIG. 25 illustrates a schematic view of a digital camera into which theimage pickup device having such a constitution as in FIG. 24 isincorporated. In FIG. 25, numeral 2701 denotes a body of camera; 2709,an eyepiece; 2711, a shatter; and 2706, a mirror.

The image pickup device of the present invention is represented bynumeral 2706, to which a light is incident through a photographicoptical system (lens) 2702 located in a lens barrel 2705. The devicegenerates a charge in each of the image pickup devices 2706 according toan subject to realize the reproducing of the subject according to thegenerated charge. The image of the subject can be reproduced on amonitoring display 2707 and memorized in a recording medium 2708 as amemory card.

The optical filter of the present invention is thinner than colorfilters comprised of general colorants so that the image pickup deviceof the present invention as disclosed here can be made with a thinthickness. As a result, the distance from the surface of the imagepickup device to the photoelectric converting portion of the imagepickup device is shortened so that the use efficiency of the light isimproved, and the sensitivity can be, therefore, improved.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application Nos.2007-184599, filed Jul. 13, 2007 and 2008-136686, filed May 26, 2008,which are hereby incorporated by reference in their entirety.

The invention claimed is:
 1. An optical filter that transmits orreflects light of a first wavelength, a transmittance of the firstwavelength being made minimal or a reflectance of the first wavelengthbeing made maximal by localized plasmons induced on a metal, the opticalfilter comprising: a substrate; and a first metal structure group inwhich a plurality of first metal structures are arrangedtwo-dimensionally in an isolated state in the in-plane direction of adielectric surface of the substrate, that is provided on the dielectricsurface of the substrate, wherein the first metal structures have afirst length in a first direction and a second length in a seconddirection orthogonal to the first direction, the first length and thesecond length being less than or equal to the first wavelength.
 2. Theoptical filter according to claim 1, wherein a period at which the firstmetal structures in the first metal structure group are arranged is lessthan or equal to the first wavelength.
 3. The optical filter accordingto claim 1, wherein the first length and the second length are the same.4. The optical filter according to claim 3, wherein the first metalstructures are in a square shape.
 5. The optical filter according toclaim 1, wherein a thickness of the first metal structures is less thanor equal to the first wavelength.
 6. The optical filter according toclaim 1, wherein the first metal structures consist of aluminium, or analloy or a mixture including aluminium.
 7. The optical filter accordingto claim 1, wherein a dielectric constant of the dielectric surface ofthe substrate and a dielectric constant of a dielectric layer of theoptical filter are the same.
 8. The optical filter according to claim 1,wherein the dielectric surface of the substrate is comprised of any oneselected from the group consisting of silicon dioxide, titanium dioxide,and silicon nitride.
 9. The optical filter according to claim 1, whereinthe first length and the second length are within a range of 110 nm ormore and 160 nm or less, a thickness of the first metal structures iswithin a range of 10 nm or more and 100 nm or less, a period at whichthe first metal structures are arranged is within a range of 340 nm ormore and 450 nm or less, and the first wavelength is within a range of550 nm or more and less than 650 nm.
 10. The optical filter according toclaim 1, wherein the first length and the second length are within arange of 90 nm or more and less than 130 nm, a thickness of the firstmetal structures is within a range of 10 nm or more and 100 nm or less,a period at which the first metal structures are arranged is within arange of 260 nm or more and 340 nm or less, and the first wavelength iswithin a range of 450 nm or more and less than 550 nm.
 11. The opticalfilter according to claim 1, wherein the first length and the secondlength are within a range of 60 nm or more and less than 100 nm, athickness of the first metal structures is within a range of 10 nm ormore and 100 nm or less, a period at which the first metal structuresare arranged is within a range of 180 nm or more and 280 nm or less, andthe first wavelength is within a range of 350 nm or more and less than450 nm.
 12. The optical filter according to claim 1, wherein the opticalfilter comprises two or more of the first metal structure groups in anin-plane direction of the dielectric surface of the substrate, andwherein periods at which the first metal structures comprising the twoor more first metal structure groups are arranged are different fromeach other, and the two or more first metal structure groups arearranged in different regions of the dielectric surface of thesubstrate.
 13. The optical filter according to claim 1, furthercomprising, in addition to the first metal structure group, a secondmetal structure group in which a plurality of second metal structuresare arranged two-dimensionally in an isolated state in an in-planedirection of the dielectric surface of the substrate, wherein the secondmetal structures have a third length in the first direction and a fourthlength in the second direction, and the third length and the fourthlength are less than or equal to a second wavelength different from thefirst wavelength, wherein the third length is different from the firstlength, or the fourth length is different from the second length,wherein the first metal structure group and the second metal structuregroup are arranged in different regions of the dielectric surface of thesubstrate, and wherein a transmittance of the second wavelength is mademinimal or a reflectance of the second wavelength is made maximal bylocalized surface plasmons induced on a surface of the second metalstructures.
 14. The optical filter according to claim 1, wherein theoptical filter comprises two or more of the first metal structure groupsin an in-plane direction of the dielectric surface of the substrate, andwherein the two or more first metal structure groups are arranged inoverlapping regions.
 15. The optical filter according to claim 1,wherein the optical filter comprises two or more of the first metalstructure groups in an in-plane direction of the dielectric surface ofthe substrate, wherein periods at which the first metal structurescomprising the two or more first metal structure groups are arranged aredifferent from each other, and wherein the two or more first metalstructure groups are arranged in overlapping regions.
 16. Alight-detecting device comprising a photoelectric converting portion, anelectron circuit, and the optical filter according to claim
 1. 17. Animage capturing device comprising a pixel area, a scanning circuit, andthe light-detecting device according to claim
 16. 18. A cameracomprising a lens, a recording medium and the image capturing deviceaccording to claim
 17. 19. The optical filter according to claim 1,wherein the optical filter further comprising a dielectric layer withwhich the first metal structure group is covered.
 20. The optical filteraccording to claim 19, wherein a distance from the surface of thedielectric layer to the surface of the first metal structures is lessthan or equal to a value d expressed by the following formula:$d = \frac{\lambda_{res}^{2}}{2n\;\Delta\;\lambda_{FW}}$ in whichλ_(res) denotes a plasmon resonance wavelength of the first metalstructures, n denotes a refractive index of the dielectric layer, andλλ_(FW) denotes a full width at half maximum of a resonance spectrum ofthe first metal structure.
 21. The optical filter according to claim 20,wherein a distance from the surface of the dielectric layer to thesurface of the first metal structures is less than or equal to a value dexpressed by the following formula:$d = \frac{\lambda_{res} - {\Delta\lambda}_{HW}}{{2n}\;}$ in whichλ_(res) denotes a plasmon resonance wavelength of the first metalstructures, n denotes a refractive index of the dielectric layer, andΔλ_(HW) denotes a half width at half maximum of a resonance spectrum ofthe first metal structure.